audio – Page 3 – Engineering Radio

MPX over IP

In the progression from Circuit Switched Data to Packet Switched Data, I can think of many different applications for something like this:

The FMC01 MPX to IP encoder can be used for multi-point distribution (multi-frequency or same-frequency network) of FM Composite audio, or as a backup solution over a LAN bridge, LAN extension, or public network. I can think of several advantages of using this for a backup when composite analog STLs are in use. There are many compelling reasons to extend the LAN to the transmitter site these days; Transmitter control and monitoring, security cameras, office phone system extensions, internet access, backup audio, etc. I would think, any type of critical infrastructure (e.g. STL) over a wireless IP LAN extension should be over a licensed system. In the United States, the 3.6 GHz WLAN (802.11y) requires coordination and licensing, however, the way the rules are set up, the licensing process is greatly simplified over FCC Part 74 or 101 applications.

Another similar CODEC is the Sigmacom Broadcast EtherMPX.

Features include:
• Transparent Analog or Digital MPX (MPX over AES), or two discrete L/R channels (analog or AES).
• Built-in MPX SFN support with PTP sync (up to 6.000km in the basic version). No GPS receivers!
• Unicast or Multicast operation to feed an unlimited number of FM transmitters with MPX from one encoder.
• Linear uncompressed PCM 24-bit audio.
• Very low audio latency: 2,5mS in MPX mode.
• Perfect match with Sigmacom DDS-30 Exciter with Digital MPX input.
• Can be used with high-quality 802.11a/n Ethernet links.
• DC coupled, balanced Analog inputs & outputs with -130dBc noise floor.
• No modulation overshoots due to compression or AC capacitor coupling.
• Decoder provides simultaneous Analog & Digital output for transmitter redundancy.
• Aux RS232 serial transparent link, Studio to Transmitter.
• Auto switchover to Analog input when Digital signal is lost.
• Centralized remote control & management software

One last thought; separating the CODEC from the radio seems to be a good idea. It allows for greater flexibility and redundancy. Using an MPX-type STL allows sensitive air chain processing equipment to be installed at the studio instead of the transmitter site.

What bitrate is needed to sound like analog FM?

As it turns out, 300 kbp/s or greater. At least in critical listening environments according to the paper titled Perceived Audio Quality of Realistic FM and DAB+ Radio Broadcasting Systems (.pdf) published by the Journal of the Audio Engineering Society. This work was done by a group in Sweden that made various observations with different program material and listening subjects. Each person was given a sample of analog FM audio to listen to, then they listened to various audio selections which were using bit reduction algorithms (AKA CODEC or Compression) and graded each one. The methodology is very thorough and there is little left for subjective interpretation.

In less critical listening environments, bit rates of 160-192 kbp/s will work.

I made a chart and added HD Radio’s proprietary CODEC HDC, which is similar to, but not compatible with AAC:

System	Codec	Bit Rate (kbp/s)
HD Radio FM; HD1 channel*	HDC (similar to AAC)	96 – 144
HD Radio FM; HD2 channel*	HDC	24-48
HD Radio FM; HD3 channel*	HDC	24-48
HD Radio AM*	HDC	20-60
DRM30 (MF-HF)	AAC/HE-AAC	34-72
DRM+ (VHF)	AAC/HE-AAC	700
DAB+	AAC/HE-AAC	32 – 128
DAB	MPEG II, Dolby Digital	192 – 256
Blu-ray	PCM**	≥6 Mbp/s
DVD	PCM, DTS, Dolby Digital	>800
CD-A	PCM	1,411
Web Streaming	MPEG I,II,III, WMA, AAC, etc	32-320, 128 typical
iTunes	AAC	128 – 256
Spotify	Ogg Vorbis	96 – 320
Wimp	AAC/HE-AAC	64 – 256

*Hybrid mode
**PCM: uncompressed data

This is the composite Mean Basic Audio Quality and 95% confidence intervals for the system across all excerpts:

Over the years, we have simply become accustomed to and now accept low-quality audio from mp3 files being played over cheap computer speakers or through cheap ear buds. Does this make it right? In our drive to take something good and make it better, perhaps it should be, you know: Better.

Special thanks to Trevor from Surrey Electronics Limited.

New Studio Project, Part II

The finished product:

This is the finished product from an earlier post. Currently, it is the studio for WAJZ in Albany, but that is not permanent. The SAS studio goes together fairly quickly, as most of the trunking between the TOC and studio is done over the SAS data channel.

The studio monitors (Tanoy Reveal) are set on little posts under the computer screens. I like this setup as the DJs are less likely to rock the house if they decide to crank up the volume on their favorite tune. I am also kind of digging the lack of a tabletop equipment pod. That takes up a lot of countertop space and always seems to be in the way. There are two CD players racks mounted below the counter (lower left), which are almost never used.

WDST, Woodstock, New York

WDST is a well-known radio station in Woodstock, NY. Formatically, I would call it Adult Album Alternative (AAA) and it is one of my favorite stations to listen to. We also do the engineering work for this station. While I was there last week, I snapped a few pictures of the studios:

All of the studio use Audioarts R-60 consoles, which are in good condition considering their age. Lots of guest microphones and the windows look out into a performance venue.

WDST music library, located in hallway outside of studio — WDST music library, located in the hallway outside of the studio

The music library is extensive.

The production room, another R-60 console. I don’t know where the microphone disappeared to, perhaps it was borrowed by the morning show.

Technical Operation Center (TOC). WDST uses NextGen from RCS for music storage, playback and automation. Other equipment includes ISDN, POTS phone, Distribution Amps, Limiters, streaming computer, STL, etc.

WDST transmitter, Broadcast Electronics FM5C

The transmitter site is on Hallihan hill, across the street from the old ATT long lines site. The station uses a Broadcast Electronics FM5C transmitter.

Forward power, almost five whole kilowatts of flame throwing power.

WDST antenna, Hallihan Hill, Kingston, NY

The antenna is a Shively 6810 2-bay half wave spaced.